Control tubing



United States Patent 1,981,863 Harris 285/133 2,054,859 9/1936 Kitching 285/133 2,494,803 l/1950 Frost et al.... 285/133 2,850,264 9/1958 Grable 285/133 3,208,539 9/1965 Henderson 175/215 3,265,091 8/1966 DeJarnett.... 138/114 3,297,100 1/1967 Crews 285/133X 2,838,074 6/1958 Lauck 285/133 3,221,826 12/1965 Webb 175/297 FOREIGN PATENTS 21,958 6/1956 Germany 138/114 1,276,007 10/1961 France 285/133 Primary Examiner-Dave W. Arola AttorneysMurray Robinson, Ned L. Conley, James A.

Bargfrede, Robert W. B. Dickerson and Bill B, Berryhill ABSTRACT: That portion of well tubing extending from surface to below mudline of offshore well includes flow passage-5' for control fluid normally maintaining in open position a,

check valve that closes in well if water level part of tubing is accidentally broken off, e.g. by ice or shipping. The flow passage is located between concentric tubular members which are rigidly connected to each other at each end.

Patented Nov. 24, 1970 3,542,40%

Sheet 2 of 5 ATTORNEY Patnted 0v. 24, 1976 352,404

Sheet 3 off:

ATTORNEY Patented Nov. 24, 1970 7 2,404

Sheet 4 of 5 V I /20 J ATTOR/VEV Patented Nov. 24, 1970 ATTORNEY CONTROL 'runrno BACKGROUND OF INVENTION The invention relates to well completion. Accordingto the prior art a separate small control pipe was disposed outside the well tubing parallel thereto extending from a source-of pressure fluid at the surface to a check valve. in the tubing at a level below the mudline. Difficulty is experienced with this construction when the tubing string is manipulated axially or rotationally during well completion, e.g. to set apacker. In the case of dual parallel string completions difficulty is experienced due to the control pipe interfering with passage of the second tubing string.

It has been suggested that concentric drill pipe, e.g. as. shown in US. Pat. Nos. 2,850,264Grable and, 3,208,539- Henderson, could be used in place of the regular tubing for the portion of the string extending down to the level of the check valve, the annulus of the dual drill pipe carrying the control fluid, e.g. oil or air under pressure. However space and strength limitations make the use of conventional dual drill pipe impractical for this purpose.

Summary of Invention According to the invention the inner and outer tubes, including both the bodies and the connecters at the ends, are combined in such a manner that the two tubes share the load, including axial tension, axial compression, torsion and bending, and the overall diameter is reduced, e.g. as compared to dual drill pipe, by reducing or omitting any annulus between the tubes, especially at the end connections.

Brief Description ofDrawings FIG. 3 is a fragmentary axial section through the connected ends of two lengths of control tubing embodying the invention;

FIGS. 2, 3. and 4 are sections taken at planes 2-2, 3-3, 4-4 of FIG. 1;

FIG. 5 is a fragmentary, axial sectional view through the connected ends of two lengths of control tubing illustrating a modification of the invention;

FIGS. 6, 7, and 8 are transverse sections taken at planes 66, 7-7, and 8-8 of FIG. 5;

FIG. 9 is a fragmentary axial section through the connected ends of two lengths of control tubing according to a further modification of the invention;

FIGS. 10 and ill are transverse sections taken at planes 10-10 and 1ll-11 ofFIG. 9',

FIG. 12 and 13 are fragmentary axial sections through the upper and lower ends of a string of control tubing embodying the invention in accordance with the second described embodiment thereof showing the connection at the upper end to a tubing hanger and at the lower end to a safety valve; and

FIG. I4 is a transverse section through parallel strings of control tubing in accordance with the second described embodiment showing'the relationship of tubing and casing.

DESCRIPTION OF Preferred Embodiments a. First Embodiment Referring now to FIG. 1 there are shown portions of two lengths of control tubing, each of which includes an inner tube and an outer tube 21 concentrically disposed therearound. As best shown in FIG. 2-, there is a narrow annulus 22 between the tubes.

Referring again to FIG. 1 the lower end of inner tube 20 is externally tapered and threaded externally to form a threaded pin connecter 23. The lower end of the outer tube 21 is provided with a cylindrical land 24 slightly smaller than the outer pipe in outer diameter. Two O-rings 25, 26 of elastomeric sealing material are carried in annular grooves in the land. The land provides the spigot ofa bell and spigot-type connectionv At the upper end ofeach length of control tubing, the outer tube 21 has a reduced end 30. A sleeve 31 has a socket 32 at one end which is snugly received on end 30 of the outer tube. The sleeve is welded'to theouter tube at 33. The upper end 34 of the sleeve is machined internally to provide a smooth cylin drical bore. End 34 forms the bell ofa bell and spigot connection. The bell end 34 is adapted to make a slip fit with the spigot provided by land 24 and to seal with O-rings 25, 26.

At its upper end 40 each inner tube 20 is tapered and threaded on its exterior. A coupling ll is internally tapered and threaded at its upper and lower ends 42, 43. Lower end 43 is bucked tight onto the upper end 40 of the inner tube. The upper end 42 of the coupling provides the box of a threaded pin and box connection and is adapted. to be made up with and broken apart from the threaded pin 23 at the lower end of an inner tube..

As best shown in FIG. 3, the interior of sleeve 31 is in contact with the exteriorof coupling 41, preferably with an interference fit such as a shrinkfit. To provide for fluid passage the exterior of coupling 41 is provided with a plurality of circumferentially spaced-apart, axially extending flats 48. The flats may be milled. A plurality of fluid passages 49 are thus provided between the coupling and sleeve, these fluid passages communicating at their lower ends with annulus 22.

The lower end of inner tube 20 is externally upset at 50 to bring it into slip fit contact with the interior of outer tube 21, the latter being internally upset slightly at 52. The extremities of the lower ends of the tubes 20, 21 are straight threaded and screwed together as shown at 53, or alternatively the lower ends of the tubes may be welded together at 54. As best shown in FIG. 4, the exterior of tube 20 is provided with a plurality of circumferentially spaced longitudinally extending flats 55 on the externally upset portion 50, the flats continuing through the threaded area 53 or weld area 54 and forming fluid passages 56 between the inner tube and outer tube. Each passage 56 communicates with annulus 22 at its upper end and at its lower end with annular space 57 which in turn is in communication with the upper ends of passages 49.

The control tubing may be assembled by heating the sleeve 31 to permit entrance of coupling 41, and then inserting tube 20 into tube 21, accompanied by rotation to make up threads 53 if used, or followed by welding at 54. On cooling of sleeves 31, the inner tube and outer tube are thus rigidly connected together at both ends so that both tubes take axial tension, axial compression, torsion and bending moment applied to the tubing, except at 59, which has the full thickness of standard tubing, and even here the outer tubing takes some ofthe bending load.

When lengths of the control tubing are to be connected together to form a tubing string, a pin 23 is tightly screwed into box 42 of another length of tubing, forming a seal therewith. At the same time the spigot formed by land 24 enters hell 34- and seals therewith. It is to be noted that the tongs used in screwing the connection together are applied to the outer tubes even though it is the inner tubes that are being screwed together, the two tubes being connected together suf ficiently strongly to transmit the torque.

b. Second Embodiment To reduce the external diameter of the control tubing as compared to the embodimentjust described, the construction shown in FIGS. 5 through 8 may be used.

Referring to FIG. 5 there are shown the connected ends of two lengths of control tubing each including an inner tube and and outer tube I21. As shown in FIG. 6, the outer periphery of the inner tube is spaced from the inner periphery of the outer tube, leaving an annulus I22 therebetween.

The lower end'of the inner tube is externally threaded to form a connecter pin 123. The lower end of the outer tube is internally upset and externally machined down to provide a smooth cylindrical land 124 almost as small as the inner periphery of the main body of tube 121. The lower end of the outer tube thus forms the spigot of a bell and spigot connection. Two O-rings 125, 126 of elastorneric sealing material are carried in annular grooves in the land I24.

The upper end of the outer tube of each length of control tubing provides a bell 134, the upper end being internally machined to form a smooth cylindrical bore adapted to make a slip fit with a land 124 and to seal with O-rings 125, 126.

The upper end of each inner tube is externally upset and internally tapered and threaded to provide the box 142 of a rolling after the tubes are positioned one inside the other. Axially extending, circumferentially spaced-apart grooves 147 on the outer periphery of tube 120 transect annular groove 145, the grooves 147 being somewhat deeper so as to provide fluid passages between the offset 146 and the exterior ofinner tube 120.

As best shown in FIG. 7, in order to provide for fluid passage the box 142 is provided on its exterior with a plurality of circumferentially spaced-apart, axially extending flats 148, which may be formed by milling. A plurality of fluid passages 149 are thus provided between the bell and box, these fluid passages communicating at their lower ends with annulus 122 through grooves 147.

The lower end of inner tube 120 has a cylindrical land 150 of slightly larger diameter than the threaded pin 123 but smaller in diameter than the outer diameter of the main body of tube 120, forming shoulders 151, 152. Land 150 is in contact with the inner periphery of internal upset 153 on the lower end of the outer tube 121, preferably making an interference fit therewith, such as a shrink fit.

As best shown in FIG. 8, the land 150 is provided with a plurality of longitudinally extending circumferentially spaced grooves 155, extending axially past shoulder 152 (see FIG. 5) in the main body portion of inner tube 120, forming fluid passages 156 between the inner tube and the outer tube. Each passage 156 communicates with annulus 122 at its upper end and at its lower end with the annular space 157 which in turn is in communication with the upper ends ofpassages 149.

The control tubing of this embodiment may be assembled by heating the ends of the outer tube and inserting the inner tube until shoulder 152 on the inner tube engages shoulder 158 of the outer tube and then allowing the outer tube to cool. Then offset 146 is rolled to engage groove 145. The two tubes are thus rigidly connected together at both ends and both take the axial tension, axial compression, torsion and bending moment applied to the tubing, except at point 159 which has the thickness of standard tubing, and even at 159 the outer tubing takes some ofthe bending load.

When the lengths of the control tubing are to be connected together to form a tubing string, pin 123 is tightly screwed into box 142 of another length of tubing, forming a seal therewith. At the same time, spigot 124 enters bell 134 and seals therewith. The tongs used in screwing the connection together are applied to the outer tubes even though it is the inner tubes that are being screwed together, the two tubes being connected together sufficiently strongly to transmit the torque.

As compared with the first embodiment it will be noted that the second embodiment is externally flush, thereby allowing a closer positioning ofparallel tubing strings in multiple completions, and allowing for larger tubing or smaller casing in both single and multiple completions.

The constructions of both the first and second embodiments are internally flush to allow passage oftools, e.g. a wire line for insertion and removal of the safety check valve.

c. Third Embodiment To effect a further reduction in external diameter of the control tubing the construction shown in FIGS. 9-11 may be used. 4

Referring to FIG. 9, there areshown the connected ends of two lengths of control tubing each including an innertube 220 and an outer tube 221. As shown in FIG. 10, the outer periphery of the inner .tube is in contact with the inner periphery of the outer tube, except where the inner tube is provided with axially extending, circumferentially spacedapart flats 219 forming fluid passages 222 extending the length of the tubing.

If the tubes 220, 221 fit pressure tight, so as to confine the fluid pressure between them to the area of passages 222, the thickness of the outer tube may be reduced compared to that which would be required if the entire area of the outer tube were subject to the bursting pressure as in the first two described embodiments. To affect such a seal over the area of contact ofthe twotubes, a pressure fit may be used. that is, the contacting surfaces of the two tubes may be placed in radial compression. A variety of ways are open to effect such a pres sure tit, rag. shrinking the outer tube onto the inner tube, or mechanically working the outer tube against thc'inner tube by cold drawing. In the latter connection it may be mentioned that the flats 219 on the inner tube may also be formed by cold drawing as well as by milling. I

The lower end of the inner tube 220 is externally tapered and threaded to form a threaded pin connecter 223. The lower end of the outer tube is provided with a slightly tapered land 224 of a mean diameter about midway between the inner and outer diameters of the main body portion of outer tube 221. An elastomeric material O-ring 225 is received in a groove in land 224. The land provides the spigot of a tapered bell and spigot type connection.

At the upper end of each length of the control tubing, the outer tube 221 has a tapered land 230 duplicating land 224. An elastomeric sealing material O-ring 233 duplicating O-ring 225 is carried in an annular groove in land 230.

A sleeve 231 has a smooth-tapered socket 232 on its lower end which makes a pressure fit with land 230, and also seals with 0-ring 233, thereby to effect a high-pressure seal between the sleeve and outer tube. The upper end 234 of sleeve 231 has a smooth-tapered socket 235, duplicating socket 232. End 234 forms the bell ofa tapered bell and spigot connection. Socket 235 is adapted to make a pressure fit with land 224 and also to seal with O-ring 225, thereby to effect a high-pressure seal.

At its upper end 240 each inner tube 220 is externally tapered and threaded. A coupling 241 is internally tapered and threaded at its upper and lower ends 242, 243. Lower end 243 is bucked tight onto the upper end 240 of the inner tube. The upper end 242 of the coupling provides the box of a threaded pin and box connecter and is adapted to be made up with and broken apart from the threaded pin 223 at the lower end of an inner tube.

As best shown in FIG. 11, the interior of sleeve 231 is in contact with the exterior of coupling 241, preferably with an interference fit, such as a shrink fit. To provide for fluid passage the exterior of coupling 241 is provided with a plurality of circumferentially spaced-apart, axially extending flats 248. The flats may be milled. A plurality of fluid passages 249 are thus provided between the coupling and sleeve. Passages 249 communicate at their lower ends with annular space 260, and space 260 is in turn in communication with passages 222. When lengths of control tubing are connected together as shown in FIG. 9, an annular space 261 is formed, duplicating space 260. The upper ends of passages 249 then are in com munication with space 260 which in turn is in communication with the lower ends of passages 222.

In connecting together lengths of control tubing as shown in FIG. 9, the threaded pin and box connection 223, 242 is made up tight enough to form a high pressure seal therebetween and also to cause tapered land 224 to enter tapered socket 235 far enough to form a high-pressure seal. In making up or breaking out the connection, the tongs are applied to the outer tubes even though it is the inner tubes that are being screwed together. The connection between the two tubes provided by the pressure fit throughout the length of the two tubes and between the sleeve and coupling is strong enough to transmit the torque required for such makeup and breakout. The two tubes are also sufficiently connected together to share other tension, compression, torsion, and bending loads, both axial and transverse, cxceptat areas 270, 271 where the inner tubing, being of a strength equal to the strength of standard tubing, is strong enough to take all the load, although even here the outer sleeve 231 takes some of the bending moment and axial compression. if need be, special high-tensile-strengtb steel may be used for the control tubing, especially the inner tube, if great lengths and weights of tubing are to be hung from the control tubing and wall thickness cannot be increased because ofdiameter limitations.

As will be apparent from FIGS. 1, 5, and 9, the axial extents of the pin portions in contact with the spigots and of the box portions in contact with the bells exceeds the radius of the areas of contact, providing sufficient length of contact to give axial stability to the parts so positioned. Likewise, as will be apparent from FIGS. 3 and 4, 7 and 8, and 10 and 11, the circumferential extents of the areas of contact of said portions exceeds 180, providing circumferential stability.

d. Connections to Hanger and Valve FIGS. 12 and 13 together form a fragmentary axial section through a string of control tubing showing in FIG. 12 the connection to the tubing hanger and in FIG. 13 the connection to the safety valve.

Referring to FIG 12 there is shown a tubing hanger 300, which may for example be a type AP mandrel hanger sold by Cameron Iron Works as illustrated on page I 125 of the I966- -67 edition of the Composite Catalog of Oil Field Equipment and Services. The hanger 300 includes a fluid passage 301 extending axially therethrough. Near the lower end of passage 301, it is tapered and threaded to form a box 302. The lowermost part ofthe passage 301 is modified from the regular hanger to include a portion of enlarged diameter providing a smooth cylindrical socket 303. Between the upper end of socket 303 and the lower end of box 302 there is an annular internal groove 304.

Fluid under pressure from a suitable source not shown is to be connected to the upper end of each of one or more fluid passages 305 extending axially of the hanger and terminating at their lower ends at groove 304.

A hanger adapter includes a nipple 310, tapered and threaded at its upper and lower ends 311, 312, and having a sleeve 313 shrink fitted to its outer periphery. Similar to the construction shown in FIG. 8 at 155, 156 the nipple 310 is provided on its outer periphery with a plurality of axially extending circumferentially spaced apart grooves 315. Fluid passages 316 are thus formed between the grooves in the nipple and the inner periphery of sleeve 313. Fluid passages 316 communicate at their upper ends with the annular space 317 formed between nipple 310 and groove 304 in the hanger and hence are in communication with fluid passages 305,

The outer periphery of sleeve 313 is provided with smooth lands 319, 320 at its upper and lower ends, each receiving a pair of elastomeric settling material O-rings 321, 322, and 323, 324 in annular grooves formed therein. When the threaded end 311 of nipple 310 is screwed into hanger 300, land 319 slips into socket 303 in the bottom of the hanger. The nipple 310 is screwed into the hanger tightly so as to seal and the 0- rings 321, 322 seal between the hanger and sleeve 313.

The lower end of the hanger adapter presents the same appearance as the lower end of a length of control tubing of the type shown in FIG. 5, so that the upper end of a length of such control tubing can be readily connected thereto in fluid-tight relationship. The upper end of such a length of control tubing is shown in FIG. 12 to include inner tube 120 and outer tube 121, the fluid passages 149 in the control tubing communicating through annular space 157 with the lower ends of fluid passages 316.

Referring now to FIG. 13 there is shown a landing nipple 400 adapted to receive a wire line insertable and removable safety valve (not shown). The landing nipple and valve may be of the type sold by Otis Engineering Corporation and illustrated at page 3841 of the l966-67 edition of the Composite Catalogue of Oil Field Equipment and Services. The landing nipple is modified from the regular Otis landing nipple by removal of the external control line that was connected to port 401 and the addition ofa smooth cylindrical land 002. A plurality of elastomeric sealing material O-rings 403, 400 are received in annular grooves in land 402.

A flow passage 406 extends through the landing nipple. The upper end 407 of the nipple is externally tapered and threaded.

A valve adapter includes a coupling 420 internally tapered and threaded at its upper and lower ends $21,422. and having a sleeve 423 shrink fitted to its outer periphery. A plurality of axially extending. azimuthally spaced-apart flats 1241 are milled or otherwise formed on the coupling 320. forming fluid passages 425 between the coupling; and sleeve, similar to passages 1 -19 shown in FIG. 7. Sleeve 423 extends axially above and below coupling 420. The lower end 426 of sleeve 423 forms a smooth cylindrical socket that slips over land 40?; and seals therewith and with O-rings 403, 4104. Fluid passages 425 communicate at their lower ends with the annulus 427 between sleeve 423 and landing nipple 400, the annulus in turn being in communication with port 101 leading into the fluid pressure bias chamber ofthe safety valve (not shown).

The upper end 430 ofthe sleeve 4123 also provides a smooth cylindrical socket. Together, the upper ends 421 and 430 of the coupling and sleeve of the valve adapter present the same appearance as the upper end of a length of control tubing of the type shown in FIG. 5 so that the upper end of the valve adapter is adapted to be connected to a length of such control tubing in fluid-tight relationship. When so connected, the fluid passages 425 communicate at their upper ends through space 157 and fluid passages 156 with the annulus 122 between inner tube and outer tube 121.

2. Parallel String Completion Referring now to FIG. 14, there is shown the disposition of two parallel strings of control tubing within the outline of the inner periphery 500 0s a well casing. The control tubing illustrated is that of the type shown in FIG. 5, each tubing including an inner tube 120 and an outer tube 121 with annulus 122 therebetween. The close proximity ofthe tubings and casing is apparent, from which it follows that the reduction in external diameter of the tubings is a highly desirable result, as well as elimination of separate pilot tubings. As is apparent from FIG. 3 (between section lines 3-3 and 4-4), from FIG. 5 (between section lines 7-7 and 8-8), and FIG. 11 (between section lines 10-10 and 11-11), the spigot end of the outer tube is alined with the box end of the inner tube, said alined spigot end and box end being sandwiched between the pin end of the inner tube on the inside and the bell end of the outer tube on the outside, thus minimizing the outer diameter ofthe overall connection as is desired. This sandwich effect occurs at the contact areas where the alined spigot and box ends are in contact with the bell end therearound and the pin end thcrewithin, providing strength as well as axial and circumferential stability of the overall joint as aforesaid.

While a preferred embodiment of the invention has been shown and described, many modifications thereof can be made by one skilled in the art without departing from the spirit ofthe invention.

We claim:

1. Control tubing comprising:

an inner tube and an outer tube disposed around the inner tube;

said inner tube having a tapered. threaded pin at one end and a tapered threaded box at the other end, said outer tube having a spigot at one end and a bell at the other end;

fluid passage means extending from one end of the tubing to the other between the inner and outer tubes;

a portion of the outer periphery of the inner tube adjacent said pin being in contact with the inner periphery of said spigot, the outer periphery of the box being in contact with a portion of the inner periphery of said outer tube adjacent said bell;

said bell projecting axially beyond said box, said pin projecting axially beyond said spigot;

said tubes being rigidly connected together one to the other at each end so as to share tension, compression, torsion, and bending moment applied to the tubing;

said portion of the outer tube adjacent said bell being connected to the box of the inner tube by a shrink fit and the portion of said fluid passage means therebetween being formed by axially. extending areas reduced below the otherwise generally cylindrical outer periphery of the box; and

said outer tube including an annular offset at its bell end pressed into an annular groove in the outer periphery of the inner tube and said fluid passage means including an axially extending groove in the inner tube deeper than said annular groove and transecting same.

2. Control tubing comprising:

an inner tube and an outer tube disposed around the inner tube;

said inner tube having a tapered threaded pin at one end and a tapered threaded box at the other end, said outer tube having a spigot at one end and a bell at the other end;

fluid passage means extending from one end ofthe tubing to the other between the inner and outer tubes;

a portion of the outerperiphery of the inner tube adjacent said pin being in contact with the inner periphery of said spigot, the outer periphery of the box being in contact with a portion of the inner periphery of said outer tube adjacent said bell;

said bell projecting axially beyond said box, said pin projecting axially beyond said spigot;

said tubes being rigidly connected together one to the other at each end so as to share tension, compression, torsion, and bending moment applied to the tubing;

said portion of the outer tube adjacent said bell being connected to the box of the inner tube by a shrink fit and the portion of said fluid passage means therebetween being formed by axially extending areas reduced below the otherwise generally cylindrical outer periphery of the box;

said box being provided by a coupling taper threaded internally at each end, the opposite end ofsaid inner tube from the pin end being taperthreaded externally, one end of said coupling being bucked tight on said opposite end;

said bell being provided by a sleeve slipped over the opposite end of said outer tube from said spigot, said sleeve being welded to said opposite end; and

said shrink fit beingbetween said sleeve and said coupling.

3. Control tubing comprising:

an inner tube and an outer tube disposed around the inner tube;

said inner tube having a tapered threadcd'pin at one end and a tapered threaded box at the other end, said outer tube having a spigot at one end and a bell at the other end;

fluid passage means extending from one end of the tubing to the other between the inner and outer tubes;

a portion of the outer periphery of the inner tube adjacent said pin being in contact with the inner periphery of said spigot, the outer periphery of the box being in contact with a portion of the inner periphery of said outer tube adjacent said bell;

said bell projecting axially beyond said box, said pin projecting axially beyond said spigot;

said tubes being rigidly connected together one to the other at each end so as to share tension, compression, torsion, and bending moment applied to the tubing;

said portion of the outer tube adjacent said bell being connected to the box ofthe inner tube by a shrink tit and the portion of said fluid passage means therebetween being formed by axially extending areas reduced below the otherwise generally cylindrical outer periphery of the box; and

said spigot of the outer tube being connected to said inner tube adjacent said pin by a shrink fit, and said fluid passage means including axially extending areas reduced below the otherwise generally cylindrical outer periphery of the inner tube in the area of said shrink fit and wherein the inner periphery of said outer tube is in contact with the outer periphery of said inner tube between said pin and box except where the generally cylindrical outer periphery of the inner tube is provided with axially extending means reduced below said generally cylindrical outer periphery to provide a portion of said fluid passage means between said axially extending means and the inner periphery of said outer tube.

4. Combination of claim 3 wherein said outer tube makes a pressure fit with said inner tube in the area of said contact therebetween.

5. Combination of claim 3 wherein said outer tube is sealed to the inner tube insaid area of contact therebetween to prevent the entrance of pressure fluid from the last said portion of said fluid passage means into said area of contact.

6. Combination of claim 3 wherein said axially extending means and said reduced areas are flats.

7. Control tubing comprising an inner tube, an outer tube disposed around the inner tube, said inner tube having a tapered threaded pin at one end and a tapered threaded box at the other end, said outer tube having a spigot at one end and a bell at the other end, and fluid passage means extending from one end of the tubing to the other between the inner and outer tubes, a portion of the outer periphery of the inner tube adjacent said pin being in contact with the inner periphery of said spigot, the outer periphery of the box being in contact with a portion of the inner periphery of said outer tube adjacent said bell, said bell projecting axially beyond said box, said pin projecting axially beyond said spigot, said tubes being rigidly connected together one to the other at each end so as to share tension, compression, torsion, and bending moment applied to the tubing, wherein said inner tube includes adjacent said pin a portion that is externally straight threaded and said spigot includes a portion inside said spigot that is internally straight threaded, said straight threaded portion of said inner tube being screwed into said straight threaded portion of said outer tube, said fluid passage means including an interruption of the external threads of said externally straight threaded portion of said inner tube.

8. Control tubing comprising an inner tube, an outer tube disposed around the inner tube, said inner tube having a tapered threaded pin at one end and a tapered threaded box at the other end, said outer tube having a spigot at one end and a bell at the other end and a fluid passage means extending from one end ofthe tubing to the other between the inner and outer tubes, a portion of the outer periphery of the inner tube ad jacent said pin being in contact with the inner periphery of said spigot, the outer periphery of the box being in contact with a portion of the inner periphery of said outer tube adjacent said bell, said bell projecting axially beyond said box, said pin projecting axially beyond said spigot, said tubes being rigidly connected together one to the other at each end so as to share tension, compression, torsion, and bending moment applied to the tubing, wherein the extremity of said spigot is connected to the exterior .of said inner tube adjacent the base of said tapered pin by a weld, said fluid passage means including a portion extending through said weld.

9. Control tubing comprising:

an inner tube and an outer tube disposed around the inner tube;

one end of said inner tube including an externally tapered threaded pin homogeneously integral with the adjacent portion of the inner tube, the other end of the inner tube including an interally tapered threaded box;

the portion of said inner tube between said ends constituting the body of the inner tube, said body of the inner tube being of uniform outer diameter throughout most of its length;

one end of said outer tube including a spigot homogeneously integral with the adjacent portion of the outer tube, the other end otthe outer tube including a bell;

fluid passage means extending from one end of the tubing to the other between the inner and outer tubes;

said bell projecting axially beyond said box, said pin projecting axially beyond said terminal portion ofthe spigot. the spigot end of the outer tube being longitudinally alined with the terminal portion otthe box end ofthe inner tube, said alined spigot end and box end being sandwiched between the'pin end of the inner tube on the inside and the bell end ofthe outer tube on the outside;

an integral portion of the pin end of the tube having an outer periphery of larger outer diameter than said uniform diameter of the body of the inner tube, said outer periphery of said integral portion of the pin end of the inner tube being in contact withthe inner periphery of said integral spigot end of the outer tube over an area having a circumferential extent greater than 180 and an axial extent greater than the radius of said inner periphery of said spigot end of said area, said larger diameter portion of the pin end of the tube being homogeneously integral with said tube;

said box end ofthe inner tube having a portion whose outer periphery is of larger diameter than said uniform diameter, said portion of the box end of the inner tube being in contact with a portion of the inner periphery of the bell end of said outer tube over an area having a circumferential extent greater than l80 and an axial extent to greater than the radius of said inner periphery of said bell end of said out'er tube at the last'said area, said larger diameter portion ofthe box end of the inner tube being I homogeneously integral with the part of the box end of the inner tube that is internally threaded; and said pin end and spigot end and also said box end and bell end being rigidly connected together one to the other so as to share tension, compression, torsion, and bending moment applied to the tubing.

10. Combination of claim 9 wherein a portion of the inner periphery of the bell end of the outer tube is connected to the box of the inner tube by a shrinlt fit and the portion of said fluid passage means therebetween is formed by axially extending areas reduced below the otherwise generally cylindrical outer periphery ofthe box.

11. Combination ofclaim 10 in which said box is formed integral with the adjacent part of the inner tube and said bell is formed integral with the adjacent part ofthe outer tube.

12. Combination of claim 10 in which said box is formed by a coupling internally taper threaded at each end with opposite tapers at each end, one end of said coupling being screwed onto an externally taper-threaded portion of the inner tube,

and said bell is formed by a sleeve telescoped at one end over the exterior of a portion of said outer tube and sealed therewith, said shrink fit of said bell to said box maintaining said sleeve in fixed axial position relative to said portion of the outer tube over which it is telescoped.

13. Combination of claim 9 wherein said spigot of the outer tube is connected to said inner tube adjacent said pin by a shrink fit, and said fluid passage means includes axially extending areas reduced below the otherwise generally cylindrical outer periphery of the inner tube in the area of said shrink fit. 

